How to iterate through an array of arrays to find duplicates

November 16, 2022

I have a problem while trying to iterate through an array of arrays. As it was suggested, I created two loops – outer and inner. I have to compare each item of each array with each item of other arrays to find out if duplicate are presented. Which method should I use for it?

  const arr = [
  ['a', 'b', 'm'],
  ['g', 'o', 'a', 'b'],
  ['w', 'o', 'u', 'k', 'a', 'b']
  ]

  const countTimes = arr => {
  arr.forEach(el => {
    for (k = 0; k < arr.length; k++) {
    for (i = 0; i < el.length; i++) {
       console.log(arr[k][i])
    }
   }
 })
}

 countTimes(arr)

>Solution :

You can create a simple counter object (if you know python, this is similar to collections.Counter):

class Counter extends Map {
  update(values) {
    for (let val of values)
      this.set(val, 1 + (this.get(val) ?? 0))
  }
}

ARR = [
  ['a', 'b', 'm'],
  ['g', 'o', 'a', 'b'],
  ['w', 'o', 'u', 'k', 'a', 'b']
  ]

const tally = new Counter()

for (let subArray of ARR)
  tally.update(new Set(subArray))

for (let [element, count] of tally)
  if (count === ARR.length)
    console.log(element)

count === yourArrayOfArrays.length selects elements that appear in all arrays, you can replace it with count > 1 to find any duplicates.